This invention relates to a portable gun welder control system and more particularly to a digital welder control system for the automotive industry in which the controller utilizes a microprocessor and modular interconnections.
Portable gun welders have been used for many years in the automotive indUstry, but the automation of the welding operation for assembly line work has introduced many problems in providing adequate control of the welding sequence. Previously, welding parameters such as squeeze time, weld time, heat intensity, cool period, etc., were programmed by an operator who dialed numerous sets of concentric switches and potentiometers on a control panel. The dial area of this control panel become crowded as well as introducing a possible error by the operator in setting the numerous switches and potentiometers on the control panel.
Another problem encountered by prior art controllers was in setting the welding heat intensity to a preselected value. The prior art used RC time delay which was somewhat temperature dependent so that the time delays did vary to a degree with operating temperature. Prior art controllers also used a continuously variable potentiometer which did not have precise known points corresponding to small increments of heat such as steps of 1% increment of heat. In addition, the accuracy of the heat settings achieved with the potentiometers often varied as much as .+-.10% or more depending upon the age or atmosphere in which the potentiometers operated. Even devices which use a stable oscillator as a timing reference and count by using integrated circuits to generate accurate time delays, are subject to error because the accuracy of the device depends upon a stable and precise reference frequency.
Prior art welder control systems also did little or nothing in the area of diagnosing malfunctions. Some malfunctions of the controllers are obvious and can be quickly diagnosed and remedied. However, in some instances, it is not readily apparent what exactly is wrong with the welder control system. In assembly line production, a problem which cannot be diagnosed quickly can lead to costly down time if a number of the portable gun welders are out of operation due to any number of possible malfunctions in a welding control system. For instance, the timing period in a sequence might extend beyond its normal period. One or both of the SCR's in a thyristor contactor might fail to fire causing half cycling of the welder transformer and eventual saturation and destruction of the same. A rise in the transformer temperature above its normal operating temperature would be an important factor to know. A low water flow to the thyristor contactor which should cause an overheating and failure of the same would also be an important factor to know. Moreover, a shorted SCR or an improper heat setting would be a further malfunction and error, respectively, that must be quickly diagnosed and remedied.
Another disadvantage in prior art welder control systems is that the circuit for the heat control used the zero crossing of the voltage wave form as a reference for generating the time delay signal. This technique required the controller be tuned to the power factor of the installation. Timing in these types of controllers was accomplished by taking the zero crossing of the voltage wave form with a correction for a particular power factor of the installation. Each time a controller is installed, a potentiometer on the controller is adjusted to tune the controller to the installation. Although tuning the controller is normally an adequate remedy, the power factor often varies due to a different gun configuration or to a dynamic change with a different work piece in the throat of the gun, In these instances, the heat control takes on some error because of the welder controller's inability to account for the changes in the power factor.